Missing exponent

Algebra Level 2

4 × 2 5 x = 625 0 x + 1 \large 4 \times 25^x = 6250^{x+1}

What is x x to 2 decimal places?


The answer is -1.34.

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1 solution

Take logarithms: 2 log 2 + 2 x log 5 = ( x + 1 ) ( log 2 + 5 log 5 ) ; 2\log 2 + 2x\log 5 = (x+1)(\log 2 + 5\log 5); isolate x x : ( log 2 + 3 log 5 ) x = log 2 5 log 5 ; (\log 2 + 3\log 5)x = \log 2 - 5\log 5; solve: x = log 2 5 log 5 log 2 + 3 log 5 = 3.194 2.398 = 1.332 . x = \frac{\log 2 - 5\log 5}{\log 2 + 3\log 5} = \frac{-3.194}{2.398} = -\boxed{1.332}.

(For the base of the logarithm, any value may be used. For instance, we may write x = 1 5 log 2 5 1 + 3 log 2 5 or x = 5 + log 5 2 3 + log 5 2 . x = \frac{1 - 5\log_2 5}{1 + 3\log_2 5}\ \ \ \text{or}\ \ \ x = \frac{-5 + \log_5 2}{3 + \log_5 2}. Perhaps these look prettier, but it doesn't make much difference for the solution.)

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